I've wanted to try this for a long time. I wasn't sure what it was, but had an idea -- it's basically caramel that's been agitated to create tiny crystals, turning it into a semi-firm block. It's extremely sweet, like eating frosting, with a buttery, penuche-like flavor. Thing is, I'm not a fan of overly sweet things, so it wasn't something I was going to be eating. I don't know how people feel about kids getting homemade products at Halloween, but I did it anyway and will be handing these out to the kids. I was concerned about the small quantity this made, but it's so sweet that it really has to be served in small pieces.
I looked at several recipes; they were all pretty similar, so I cobbled together my own list of ingredients then followed instructions from a couple of others. This made about 100 pieces (about the size of a Mary Jane candy). Here's what I used:
1/2 cup salted butter
3/4 cup whole milk
1/4 cup heavy cream
4½ cups granulated sugar
2 pinches of salt
pinch of cream of tartar
1 can (397g/14 oz) condensed milk
1 teaspoon vanilla extract
Grease a 13 x 9" baking pan, line with foil, then lightly grease again. In a large, heavy saucepan, combine the butter, milk, cream, sugar, and salt over medium-low heat. Stir constantly, removing any sugar crystals or splatters from the sides of your pan with the wet pastry brush (do this throughout the entire process) and slowly bring to a boil. Once the mixture is boiling and the sugar has completely dissolved, stir in the cream of tartar, then the condensed milk. Bring the mixture back to a boil over medium-low heat, still stirring continuously. Once boiling, stop stirring. Allow the mixture to cook, wiping down the pan sides as necessary with the wet pastry brush to remove any crystals. Once it reaches 240F, remove from heat and beat with a whisk for about 5-10 minutes; this will encourage proper crystallization. (I used an electric hand mixer wit the whisk attachment on the lowest speed.) Continue to mix until the mixture has thickened and cooled slightly but is still pourable. Quickly pour the mixture into your prepared pan and smooth the surface; remove any air bubbles by tapping the pan on the counter top a few times. Allow to cool for several hours before cutting (or scoring so you can break it later). To make cutting easier, lift the entire slab by the foil out of the pan and set on the counter. Store in an airtight container.
This was pretty easy to make. I had a slight issue of a spot in the bottom of my pan that cooked hotter, creating browning that pulled up and resulted in a few hard, dark pieces of candy mixed in, but I picked those out and used caution scraping the bottom of the pan. (One recipe recommends stirring occasionally while it's cooking in order to keep this from occurring. An undamaged pan shouldn't have this problem.) The only tricky part was knowing when to stop whisking after it's cooked. I whisked too long, I think, so that the mixture was cooler than it should have been when I poured it out. It was firming up quickly while I was trying to scrape the last out of the pot. Then when I tried to smooth the top, it just dragged the mixture around. On top of that, I found my counter is not level, so one side of the pan was substantially thicker than the other. Also, some areas of the candy were softer and creamier than others, which I can only assume had something to do with the cooling process since it all came from the same place. I used a slender, sharp knife to cut pieces about 1.5" x 1" x 1/4" and wrapped them in waxed paper.
Conclusion: Delicious! Tooth-achingly sweet, but nice to nibble on in tiny bits, and good with strong coffee. For an adult version, I would add rum or bourbon instead of vanilla. Coconut or nuts would be good too, as long as they don't burn. (Maybe have them in the pan already and pour the candy on top.) One could also try dipping an edge in dark chocolate.
Original recipes: Scottish tablet via Not So Humble Pie and Old English Butterscotch 1934 via The Vintage Kitchen